Solar cells rely on the semiconductor junction to convert sunlight into electricity. The junction consists at least of two layers of opposite types one layer being an n-layer with an extra concentration of negatively charged electrons and the other layer being a p-layer with an extra concentration of positively charged holes. There is at least a window layer, which is usually heavily doped and an absorber layer, which is either a lightly doped or undoped semiconductor. In solar cells, only photons that are near or above the semiconductor bandgap of the absorber layer can be absorbed and utilized. In the solar radiation, there is a limited amount of flux of photons with energy above such a value. Unfortunately, all photons will have to pass through the doped window layer before the photons reach the absorber layer. Those photons absorbed by the window layer will not be able to be converted into useful electricity and are wasted. One way to reduce such an absorption is to make the doped window layer with a wider bandgap and to make the doped window layer very thin. However, a minimum thickness is required for the doped window layer in order to maintain build-in potential. When the bandgap of the window layer is increased beyond the absorber layer, there is a mismatch in the band edge at the junction. Such a mismatch at the band edge prevents carriers, electrons or holes, to flow smoothly and get collected, which then results in poor solar cell performance, as represented often by a “roll-over” or “double-diode” effect in the current-voltage (I-V) characteristics.
As a specific example of the problem, single-junction hydrogenated amorphous silicon (a-Si) based solar cells could be fabricated. In these a-Si based solar cell (including solar cells based on a-SiGe:H alloys), the absorber layer is sandwiched between two doped layers which generate an electrical field over the intrinsic layer (i-layer). Either the n-type doped layer or the p-type doped layer could serve as the window layer, which is on the side the sunlight enters. However, due to the fact the hole mobility is much smaller than the electron mobility in a-Si based materials, the p-layer is often used as the window layer so that holes, having smaller mobility compared with electrons, will need to travel less distance to get collected. For this reason, the properties of the p-layer must meet several, often conflicting, requirements. The p-layer must have a wider bandgap so that sunlight can pass through the p-layer without being absorbed before reaching the intrinsic layer (absorber layer in this case) for the photon to electricity conversion. On the other hand, this p-layer must not have a bandgap wider than the i-layer since there would be a mismatch in the band edge at the p-i interface.
In order to make a single-junction solar cell with higher efficiency, it is desirable to reduce the bandgap of the absorber layer, for example by using alloys having a small amount (about 10-30%) of germanium. Earlier work by the inventor found that the a-SiGe solar cells with about 10-30% Ge in the i-layer is more stable after prolonged exposure in the sun. The p-layer for such a lower bandgap a-SiGe absorber layer needs to have a smaller bandgap so that the p-layer can form a smooth interface with the lower-bandgap a-SiGe i-layer while at the same time the p-layer needs to have a wider bandgap to have minimized absorption.
The problems and difficulties represented here for single-junction a-SiGe solar cell apply also to a broader range of solar cells that have at least a doped window layer and a lightly doped or undoped absorber layer.
Therefore, there is a need to design a novel window layer that overcomes most, if not all, of the preceding problems.